Nitric oxide (NO) is a major signaling molecule in biological systems. It serves in normal physiology as a vasodilator and a regulator of cellular processes including energy metabolism. It is also involved in tissue pathology through the damage caused by nitrogen related reactive oxygen species (ROS). The abundant NO synthase (NOS) found in the organ of Corti suggests a role for NO in the mechanical or electrical function of the organ. The NOS found in the stria vascularis and endothelial cells of blood vessels could be important to blood vessel permeability as well as vasodilation. The state of the cochlear blood flow (CBF) and the microcirculation in the cochlea is of critical importance to hearing. It is known that age, drugs and loud sound cause changes in cochlear circulation but it is not know whether these changes are a cause of hearing loss or perhaps accelerate declining hearing. This proposal is designed to evaluate the normal physiology of NO generation in the cochlea and the pathological consequences of ROS formation. Aim 1 has the goal of evaluating whether NO produced in the organ of Corti affects the sensitivity and mechanical responses of the organ. Electrophysiological and mechanical measurements of organ of Corti responses to sound while blocking or enhancing NO levels in the cochlea accomplish this. Aim 2 has the goals of determining: whether NO caused vasodilation of CBF reduces the damaging effects of loud sound exposure; whether NO prevents abnormal vascular permeability of the CBF; and how hypoxia alters the cellular biology of cochlear vascular smooth muscle. Mutant mice, deficient in endothelial cell NOS, will be used in part of this work. In Aim 3 it will be determined if an inducible form of NOS (iNOS) is responsible for loss of sensitivity and of sensory cells as a result of noise exposure. These studies will use the mutant mouse lacking iNOS as well as the experimental approach of pharmacological inhibition of iNOS. The work will also determine the amount and cellular locations of nitrogen related ROS. By correlation of the cellular ROS levels with the early expression of stress proteins and subsequent cell death, it will be possible learn the importance of NO in the pathological process of noise-induced hearing loss. The long term goals of this work are to learn how cochlear microcirculation and the NO biochemistry can be treated to prevent noise induced and age-related hearing loss .